Earthquakes happen everyday in Costa Rica, and that is a good thing. Costa Rica is a very active location geologically. The (normally) small quakes that occur here daily are the earth's way of relieving stress gradually. If we didn´t have many small quakes, we would eventually have a massive earthquake which would do tremendous damage and cost many lives.
Why is Costa Rica so active geologically?
A brief explanation of the earth's make up will help you understand this.
The earth is a sphere and has layers sort of like a jaw-breaker. There are four principle parts: crust, mantle, outer core, and inner core. For our explanation, you only need to understand about the crust and the mantle.
The earth's crust is a thin layer of material (compared to the actual size of the planet), rather like the peel on an apple. This layer of material is made up of rock strata and soil. It contains cracks which are called faults. The earth's crust is "floating" on the materials that make up the earth's mantle. The mantle is made up of rock type materials that are viscous. The crust is sort of like foam on top of a boiling liquid. Within the boiling liquid there are convection currents and the foam is moved across the surface by these currents. Pieces of the crust are not only floating on the mantle, they are being moved by the convection currents operating within it. These pieces of crust are called tectonic plates. There are two types of tectonic plates: Continental and Oceanic.
The Oceanic Plates are made up of dense material like basalt. The Continental Plates are made of a lighter material like granite. Although both of these plates are floating on the mantle, the continental plate, being lighter, floats more easily.
The earth has seven major plates and numerous smaller ones. Because of the mantle's convection currents these plates may be moving in one of three ways in relationship to each other. If the edges of the plates are sliding past one another you have a transform boundary like the San Andres Fault in California. Earthquakes there are caused when the two plates slide by one another. If two plates are moving away from each other (for example, one is moving east and the other west) you have a divergent boundary like the Great Rift Valley in Africa or the Mid-Atlantic Ridge. If two plates are moving toward each other you have a convergent boundary. This type of movement often happens at the edges of continentals, and this is the type of boundary found off the coast of Costa Rica.
Costa Rica is the meeting place of two major plates and a couple of smaller plates as well. The Caribbean Plate is moving west and the Cocos Plate is moving east. These plates meet head on just off the Pacific coast of Costa Rica. The leading edge of the Caribbean Plate is made up of the lighter continental material, and the leading edge of the Cocos Plate is made up of denser oceanic material. When they meet, the oceanic material sinks beneath the continental material. The earthquakes are caused by these two plates colliding and sliding over each other. Another by-product of this collision is volcanoes. And Costa Rica has plenty of those, too. (How the volcanoes form willl be explained in the next post.) Where the plates collide there is a deep trench offshore and this trench runs along Costa Rica's coast. As the plates push at each other, sometimes massive chunks of materials will break free as they move past one another. On the surface, we feel this as an earthquake.
How much do the plates move? Scientists estimate that they move about 10 centimeters a year. That doesn't seem like much, but remember that these gigantic slabs of material are pushing at each other. Stress and strain build up and when they finally jerk past each other, earthquakes are the result.